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12
7
5 D 4
8
10
8
6
4
10
13a
14a
2
5
14b
11
6
9
13b
0
19.5
20.0
20.5
21.0
21.5
22.0
22.5
23.0
E 0-0 (T*) / 10 3 cm -1
Figure 6.4 Relationship between the experimental overall quantum yield of [Tb 2 L 3 ] helicates
in water and the energy of the ligand triplet state. Squares: dicarboxylates. Circles: phospho-
nates (ligand numbering according to Table 6.3).
easy to avoid and a protective and rigid environment will prevent solvent molecules to
quench the excited state by collisional deactivations. However, this becomes problematic
for NIR-emitting ions, particularly Nd III (gap 5400 cm 1 )andEr III (6600 cm 1 )sothat
deuteration or halogenation of ligands is required for coordination compounds to display
reasonably large quantum yields [35,36,56,57]. Alternatively, inorganic matrices are bet-
ter suited since their phonon energies can be quite low (
600 cm 1 ).
Equations 6.2 and 6.3 give a clue on one way of decreasing
<
t rad by increasing the
refractive index. This is perfectly well illustrated with helicate [Eu 2 (L7) 3 ] the radiative
lifetime of which amounts to 6.9ms in water ( n
1.33) and 4.9ms in the solid
state (Table 6.2) [47]. Taking the commonly accepted value n
¼
1.5 for solid state samples
of complexes with organic ligands, one notes, referring to Equation 6.3, that the ratio
4.9/6.9
¼
0.71 is almost identical to (1.33/1.5) 3
¼
¼
0.70. The lifetimes of the aqueous solu-
tion,
2.43(9) ms, and the solid state sample, 2.36(1) ms, are the same within exper-
imental errors so that the decrease in
t obs ¼
t rad leads to a one-third increase in Q E Eu , from 36 to
48%. Unfortunately, the sensitization efficiency decreases, from 58 to 50% so that in fine ,
the solid state sample presents only a 15% increase in overall quantum yield compared
with the solution. When the same helicate is doped into silica nanoparticles [48], analysis
of the photophysical parameters becomes more intricate because the I tot / I MD ratio
changes, which was not the case between solution (4.01) and solid state (4.14) samples.
Here this ratio amounts to 5.38 in pure silica nanoparticles and to 5.90 in silica nanopar-
ticles derivatized with amine groups for bioanalytical purposes. Therefore the refractive
index correction ( n
1.475 for pure silica nanoparticles;[58] in the original paper the
authors used 1.45[48]; data are recalculated), contributes to increase
¼
t rad by only 5% with
respect to the solid state sample while the larger I tot / I MD ratios decrease it by 25 and 32%,
respectively. The larger I tot / I MD ratios stem essentially from more intense hypersensitive
transitions, possibly reflecting a lowering of symmetry of the metal ion sites due to inter-
action of the ligand strands with silica. Indeed, a lower symmetry would favour more
 
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